The series begins with a card defining its title — “strong longing for or impulse toward wandering” — as if that word had somehow fallen out of common use, and the main point wasn’t the punning “lust” in the title. This is a show about sex — as urge, satisfaction, distraction. (And love too, but mostly about sex.) Toni Collette and Steven Mackintosh play Joy and Alan Richards, a psychotherapist and English teacher, respectively, and parents of three children, mostly grown. Having tired of each other, sexually but not personally, they decide to try other people while staying together.